U.S. Pat. No. 4,629,676 (Hayakawa et al) and European Patent Publication No. 0174634B1 disclose an image forming method comprising the steps of imagewise exposing to light a light-sensitive material, and heating the light-sensitive material. The light-sensitive material comprises a support and a light-sensitive hardening layer containing silver halide, a reducing agent and a polymerizable compound. The light-sensitive material is heated to develop the silver halide and to harden the polymerizable compound. The light-sensitive material usually contains a base precursor as a development accelerator.
U.S. Pat. Nos. 5,122,443 (Takeda), 5,290,659 (Takeda) and 5,393,651 (Hoshi) disclose embodiments of the above-mentioned light-sensitive material, which are advantageously used for the preparation of a lithographic plate. U.S. Pat. Nos. 5,304,454 (Yokoya et al), 5,326,667 (Yokoya et al) and 5,328,800 (Yokoya et al) disclose other embodiments of the light-sensitive material, which is advantageously used for the preparation of a color proof. In the light-sensitive materials for the lithographic plate or the color proof, a cross-linkable polymer can be used in place of or in addition to the polymerizable compound. Further, the light-sensitive hardening layer comprises a hardening layer containing the polymerizable compound or the cross-linkable polymer and a light-sensitive layer containing silver halide. An overcoating layer containing a base precursor may be provided on the light-sensitive layer.
As is described in U.S. Pat. No. 5,122,443 (at column 20, lines 1 to 43), a polyvinyl alcohol having a high saponification degree (preferably not less than 70%, more preferably not less than 80%, and most preferably not less than 95%) is preferably used as a hydrophilic binder contained in the light-sensitive layer or the overcoating layer. Such a polyvinyl alcohol has a function of preventing influence of oxygen in the air (which has a function of inhibiting a polymerization reaction). A polymerization reaction at heat development can smoothly proceed by the function of the polyvinyl alcohol of a high saponification degree. This effect is remarkable when the saponification degree is not less than 95%.
In addition to the polyvinyl alcohol, a surface active agent can be added to the light-sensitive layer or the overcoating layer. U.S. Pat. No. 5,122,443 describes that an ethylene oxide nonionic surface active agent can be contained in the light-sensitive layer or the overcoating layer. The ethylene oxide nonionic surface active agent can be advantageously used in a light-sensitive material containing silver halide, a reducing agent and a polymerizable compound. U.S. Pat. No. 4,956,260 (Nakamura) discloses the ethylene oxide nonionic surface active agent in more detail.